Basic Outline of Relativity

We can understand the basic outline of theory of relativity in a simple following way from Fermat’s principle of least time.

First you need to understand Fermat’s Principle of least time.

It states that the path taken between two points by “a ray of light” is the path that can be traversed in the least time. So it implies light will always travel in the shortest path between two points.

After believing in this principle, we can take an example of moving rocket with 9.8 m/s^2 of acceleration and earth.

From Newton’s Gravitational law, it was known that all objects near to the earth will fall at a rate of acceleration 9.8 m/s^2. Many of them had learnt this in their school days. Consider that the rocket is at first on earth at rest. If you are inside the rocket, you will see the things inside the rocket will fall at a rate of 9.8 m/s^2 when it is subjected to fall from a height not comparable to the radius of the earth.

[Since gravitational field of earth will be different at different places]

Now, I made you to sleep in the rocket by giving you anesthesia. After you had slept, the rocket was started and made to fly into the space with an acceleration of 9.8 m/s^2 .

From Newton’s law it is known that a man inside the rocket will not feel any difference between these two situations. So, he will think he is still at earth, since the experiments done in the rocket will be as same as it is at earth.

Now if you take torch light and pointed it on the horizontal direction, from very careful measurements you can see that light beam bends exactly same amount as it is in earth.

Since all the experiments done with other objects gives same results in earth as well as in rocket, it implies Light should also give same results independent of the place. 

Thus we came to know that Light indeed bend near the gravitational field. 

From this we can get two information.

One is,

• Newton’s Gravitational law says that only masses are attracted by gravitational force. But here light is attracted towards the earth, where light is thought to be pure energy. It shows that energy and mass has some relation.

The second information is derived from Fermat’s principle.

• If light travels in the shortest path between two points,

In horizontal direction the shortest path between the two points is the straight line drawn between those two points. But light here is actually travels in a bend path not following Fermat’s principle.

To make Fermat’s principle the true one, it was just made a small adjustment in the statement that follows as,

The shortest path between the two points in horizontal direction near the gravitational field is really the bend path.

That says that the space itself is a curved one. And light travels in the shortest path in that curved space where the path of the light is curved. Thus we can get two aspects of Relativity theory.

I am explaining here just the simple outline of Relativity theory and not the complete one.

This is to get just a little basic knowledge of Relativity theory to common people.